Crews cleaning up the Mayflower neighborhood in April, a week after the spill.

Credit Michael Hibblen/ KUAR News

ExxonMobil released a statement late Wednesday afternoon saying an independent assessment has found that "original manufacturing defects" were to blame for the March oil pipeline rupture in Mayflower.

An estimated 150,000 gallons of tar sands were spilled in the residential neighborhood.

The company said the report by the Hurst Metallurgical Laboratory, which inspected the damaged section of pipe, indicates that cracks and a lack of toughness in the 65-year-old pipeline material were among contributing factors. It said there was no evidence to indicate corrosion at the site of the spill.

ExxonMobil says it is conducting supplemental testing to determine all factors that led to the pipeline rupture.

Below is the full statement, emailed to Talk Business Arkansas, by ExxonMobil spokesman Aaron Stryk:

ExxonMobil Pipeline Company (EMPCo) and the Pipeline and Hazardous Materials Safety Administration (PHMSA) have received the results of an independent assessment conducted by Hurst Metallurgical Research Laboratory, Inc. on the Pegasus Pipeline segment that failed on March 29, 2013. We are currently in the process of reviewing and analyzing the data. Based on the metallurgical analysis, the independent laboratory concluded that the root cause of the failure can be attributed to original manufacturing defects – namely hook cracks near the seam. Additional contributing factors include atypical pipe properties, such as extremely low impact toughness and elongation properties across the ERW seam. There are no findings that indicate internal or external corrosion contributed to the failure.While we now know the root cause of the failure, we are still conducting supplemental testing, which will help us understand all factors associated with the pipe failure and allow for the verification of the integrity of the Pegasus Pipeline. These tests will help us determine the mitigation steps we need to take to ensure a similar incident does not occur again.

EARLIER REPORT: The federal agency that regulates oil pipelines in the U.S. says it has received a report looking into the cause of the Mayflower oil spill. It was also expected to include details about inspections before the spill.

Wednesday was the deadline for the report after several delays had been requested.

ExxonMobil's Pegasus pipeline ruptured March 29, spilling thousands of barrels of raw crude and forcing the evacuation of 22 residents.

In an email to KUAR News, Damon Hill with the Pipeline and Hazardous Materials Safety Administration would not share details of what was in the report and says it will not be made public at this time.

We have received the metallurgical report, and our pipeline safety officials are reviewing it. The pipeline is shut down and it will remain that way until we are convinced it can be safely restarted. The report is part of an active investigation and cannot be released at this time.

ExxonMobil says the report comes from Hurst Metallurgical Laboratory, which inspected the damaged section of pipe after it was removed from the ground. ExxonMobil says it will comment after it reviews the report.

Arkansas Attorney General Dustin McDaniel said earlier this year that the hole in the 65-year-old pipeline was 22 feet long.